The invention relates to a method and a device for controlling a NOx storage catalyst which is arranged in an exhaust gas channel of an internal combustion engine by means of a NOx-sensitive measuring device, a particular a NOx sensor, that is arranged downstream of the NOx storage catalyst.
It is known to place NOx storage catalysts in an exhaust channel of internal combustion engines for post-processing exhaust gases of temporarily lean-running internal combustion engines. These NOx storage catalysts store nitrogen oxides present in the exhaust gas during lean operating phases of the internal combustion engine. The NOx storage catalyst is periodically subjected to a NOx regeneration to restore the initial NOx storage capacity, for which purpose the catalyst is exposed to a rich exhaust gas atmosphere. Desorbed NOx is reduced by the reducing exhaust gas components carbon monoxide CO and non-burned hydrocarbons HC to nitrogen N2. It is also in necessary to perform after extended time intervals a desulphurization of the NOx storage catalyst to remove unwanted sulfate deposits. Desulphurization is typically also performed by exposing the catalyst to a rich exhaust gas while the temperature is adjusted to above 600xc2x0 C.
It is also known to control NOx storage catalysts, in particular the NOx regeneration and desulphurization of these catalysts, with NOx sensors located downstream of the NOx storage catalyst. NOx breakthrough can be detected and a NOx regeneration can be initiated by measuring the NOx concentration after the catalyst.
EP 0 916 941 A describes a method for detecting a NOx storage activity of a NOx storage catalyst with a downstream NOx sensor. After the internal combustion engine is switched into a lean mode, a signal that depends on the NOx concentration is measured and its curve form dependence analyzed to determine a reference time for monitoring the storage activity. As a criterion for determining the reference time an absolute minimum of the signal form within a predetermined wait time after switch-over of the internal combustion engine is used. In particular, the height of the signal of the NOx sensor is used to correct the offset of the sensor. WO 00/76636 describes a method for calibrating a NOx sensor, wherein likewise a minimum in the signal form after switch-over of the internal combustion engine into a lean mode is used for a correction of the offset.
The NOx storage catalyst can be controlled even more precisely if the NOx regeneration and desulphurization time intervals are not matched to an actual NOx concentration after the NOx storage catalyst, but to a quantity of NOx that had passed through the NOx storage catalyst during a lean phase of the internal combustion engine, wherein the NOx quantity can be determined by integrating the measured NOx concentration. This concept has the disadvantage that the time at which NOx is actually stored again after a regeneration phase, is unknown. The time at which the internal combustion engine is switched into the lean mode typically forms the basis for starting the NOx storage phase. This point in time, however, represents only a coarse approximation of the actual start of the storage process. For example, a time delay is ignored which represents the distance traveled by the exhaust gas between the internal combustion engine and the NOx storage catalyst. At the start of the lean phase, the NOx storage catalyst can also be in a temperature range which prevents a reliable storage of nitrous oxides. Another disadvantage is the typical response time associated with modern NOx sensors, which can cause a delay in the signal change after the NOx concentration in the vicinity of the sensor has changed. As a result, integration of the NOx mass flow typically starts too early, which entails a premature start of the regeneration, which can then adversely affect the fuel consumption. Many control processes of the NOx storage catalyst suffer from the problem that the temporal relationship between an operating point of the internal combustion engine and the behavior of the catalyst is not known.
It is therefore an object of the invention to propose a method and a device for controlling a NOx storage catalyst of the aforedescribed type, which takes into account a temporal correlation between an operating state of the internal combustion engine and the measurement from a NOx sensor located after the NOx storage catalyst.
The object is solved by the characterizing features of the method of claim 1 and the device of claim 17. According to the method of the invention, it is proposed to perform an analysis of a signal form of the NOx sensitive measuring device after the internal combustion engine is switched into a lean operating mode, for example, after regeneration of the NOx storage catalyst, with respect to at least one preset criterion. The time at which at least one of the preset criteria is satisfied, is defined as a reference time for controlling NOx regeneration and/or desulphurization processes of the NOx storage catalyst.
According to the invention, at least one of the preset criteria is a transition of a decreasing signal form to a curve with a slope of approximately zero or a positive slope of the signal form and/or a drop of the signal below a predetermined threshold value. A threshold value can be predetermined for the slope. The first criterion corresponds essentially to the presence of a minimum in the signal form of the measuring device, which can, of course, also be determined by other methods for determining a minimum, for example, by a comparison of the magnitude of sequentially measured signal values. The criteria should be preset in such a way that under standard conditions of catalyst and measuring device a reference time can always be determined, so that the control of the NOx storage catalyst can be enabled.
According to an advantageous embodiment, the signal form of the measuring device for determining the reference time is analyzed immediately at the time when the internal combustion engine is switched into a lean operating mode or only after a short delay. Starting the analysis of the signal form can also require that the NOx storage catalyst is in a temperature range that is suitable for storing NOx.
According to a particularly advantageous embodiment, the criterion to be satisfied can include a simultaneous drop of the signal below a predetermined threshold value instead of or in addition to a slope of approximately zero or a positive slope of the signal form.
Advantageously, one of the preset criteria may be that the signal level remains below a predetermined threshold value for a predetermined minimum time interval before the signal form is analyzed. This criterion can take into account signal forms which, after a regeneration of the NOx storage catalyst, have already a relative constant or low level when the internal combustion engine is switched into a lean operating mode, with the level subsequently slowly increasing.
Another criterion can be preset, whereby the measured signal drops below a predetermined threshold value during the analysis. Optionally, this may be required for a certain minimum time duration.
Regardless which of the criteria is/are selected, it can be particularly advantageous with a low signal-to-noise ratio to smooth the signal form before the analysis. This can be done, for example, by averaging adjacent signal values or by other known methods.
According to another embodiment of the invention, a maximum time interval can be defined after the start of the analysis of signal form, wherein if a reference time could not be successfully established based on the aforedescribed criteria after the maximum time interval, then the end time of the time interval is used as the reference time. According to an alternative embodiment, if the time interval has passed unsuccessfully, a renewed regeneration of the NOx storage catalyst is started, while the reference time is subsequently defined anew. It can also be advantageous to use the information about a prior unsuccessful search for a reference time for additional control processes of the internal combustion engine or the NOx storage catalyst. For example, such information can be used in a diagnostic test to describe the condition of the catalyst or to enable a lean control of the internal combustion engine.
Preferably, the analysis of the signal form can terminated immediately after the reference time has been determined, and the signal of the measuring device can be evaluated and released for controlling the NOx storage catalyst.
According to a preferred embodiment, the reference time is used as a start time for beginning the NOx storage in the NOx storage catalyst for controlling NOx regeneration and/or desulphurization of the NOx storage catalyst. Advantageously, approximately at the reference time, integration of the NOx mass flow through the NOx storage catalyst is started and/or the quantity of NOx stored in the NOx storage catalyst is calculated.
Under certain circumstances, the signal form at the reference time may be used as an offset for calibrating the measuring device. However, this requires relatively strict criteria for determining the reference time. In particular, the signal form of the measuring device should have an absolute minimum at the reference time.
For a precise and operation-friendly control of the NOx storage catalyst, the reference time is advantageously determined after each NOx regeneration and desulphurization of the NOx storage catalyst.
According to a preferred embodiment of the method, the reference time for controlling the NOx storage catalyst should be valid until the internal combustion engine is switched again into a lean operating mode or until lean exhaust gases are detected at the location of the measuring device. In this case, a reference time determined in a lean phase is therefore valid until a new search for a reference time is started in a subsequent lean phase. Alternatively, the reference time can also be valid until the internal combustion engine is switched over into a stoichiometric or rich operating mode and/or until the presence of a stoichiometric or rich exhaust gas is detected at the location of the measuring device.
A device is also provided which includes means for performing the process steps for analyzing a signal form of the NOx -sensitive measuring device after the internal combustion engine has been switched into a lean operating mode with respect to at least one preset criterion, wherein at least one of the preset criteria is a transition of a decreasing signal form to a slope of approximately zero or a positive slope of the signal form and/or a drop of the signal below a predetermined threshold value, as well as determining a time when at least one of the preset criteria is satisfied, as a reference time for controlling the NOx regeneration and/or desulphurization processes of the NOx storage catalyst.
In a preferred embodiment, these means include a control unit that stores a procedure for controlling in digital form the process steps that control a NOx storage catalyst. This control unit can be integrated in an engine controller.
Additional advantageous embodiments of the invention are recited as features in the remaining dependent claims.